Experimental Study of Graphite Dust Generated from Fuel Pebble Elements of HTRs

Zhou Hongbo1, Shen Ke1, Yu Suyuan2*

Institute of Nuclear and New Energy Technology, Tsinghua University

9/17/2014 Hangzhou

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outline

1. Fuel handling system in HTR-10

2. Graphite dust test platform by lifting fuel pebble

3. Preliminary experimental data

4. Concluding remarks

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Fuel handling system in HTR-10

Graphite serves as coating material fuel elements. These elements are pneumatically conveyed to the top of the core through stainless steel tube, and discharged out of the core under the action of gravity and inertia. Thus accomplishing on-line refueling.

In the fuel handling system, the diameter of fuel loading tube is 62mm ， and the total length is about 23m.

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There are 27,000 pebble fuel elements in the core. In normal operation, it needs to load or unload 125 elements per day.

The graphite matrix material is manufactured with a powder mixture which is composed of 64 wt.% natural flake graphite,16 wt.% artificial graphite, and 20 wt.% phenol resin binder.

The diameter of fuel element is 60mm.

Fuel handling system in HTR-10

pebble fuel elements

Graphite dust generated inevitably from abrasion in the core. Combined with fission product, the dust would became a hidden problem for safety of HTR.

There are 3 zones for the fuel elements to wear: the bottom of the core, the lifting pipe and the unloading pipe. Previous studies have shown that fuel wear more seriously in the lifting pipe than in the other 2 zones.

The wear mainly happens on the surface of the fuel pebble.

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Fuel handling system in HTR-10

Graphite dust

Choose HTR-10 as the study object, and study wear performance in the pneumatic fuel loading system, especially focusing on the impact brought by the convey speed affect the performance of its wear and tear.

The results obtained in this study will help the understanding of graphite dust generation in HTR-10 and HTR-PM.

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Fuel handling system in HTR-10

Graphite dust

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outline

1. Fuel handling system in HTR-10

2. Graphite dust test platform by lifting fuel pebble

3. Preliminary experimental data

4. Concluding remarks

Graphite dust test platform by lifting fuel pebble

The test platform consists of a stainless steel lifting pipe, gas sources and gas lines, timing system, and a graphite sphere carrying system.

The whole platform is supported by a metal framework and covers an area of about 2 m2 with

the height of 9 meters.

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The stainless steel pipe use the same material as fuel handling system in HTR-10 .

The pipe has an inner diameter of 62 mm. The straight part of the pipe is 8000 mm in length, due to space limitation of the experimental hall.

The pipe has a roundness tolerance of 0.25 mm, a straightness tolerance of Φ1mm/m.

One end of the pipe was bended into a semicircle with a radius of 300 mm. The ovality is less than 1.03, while the minimum inner diameter is not less than 61mm, in order to ensure the free passing of the graphite sphere.

test platform

Graphite dust test platform by lifting fuel pebble

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some important components of the test platform system

Solenoid valveCompressed gas

Graphite dust test platform by lifting fuel pebble

11timing system

Graphite dust test platform by lifting fuel pebble

some important components of the test platform system

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Item Specification Measurement value

Density （ g/cm3） >1.70 1.72

Total ash (ppm) ≤300 89

Li content (ppm) ≤0.3 <0.02

Impurity (ppm EBC) ≤3.0 <1.0

Thermal conductivity,1000 C (W/cm·K) ≥0.25⊥0.31∥ 0.28

Corrosion rate, 1000℃,He+1vol%H2O (mg/cm2·h)

≤1.3 0.95

Erosion rate (mg/h·fuel element) ≤6 3.2

Number of drops (4 m) ≥50 >220

Breaking loading (kN) ≥18⊥22.7∥ 21.6

Experimental graphite sphere

Graphite dust test platform by lifting fuel pebble

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Compressed air, nitrogen and helium were used as gas source. Different average lifting velocities were obtained by adjusting the pressure at the outlet of pressure reducing valves.

Since each lifting results in only a very small amount of mass loss, the graphite sphere was lifted for 10 times, and the weight of it was measured before and after the test to calculate the abrasion rate.

Before weighing, the spheres was wiped by ethanol and then dried at 110 C for 15 min.

The abrasion rate can be calculated by dividing the mass loss by 8.942 m, which is the total distance of the graphite sphere. Therefore, the abrasion rate and mean speed relationship can be obtained.

Determination of abrasion rate

Graphite dust test platform by lifting fuel pebble

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outline

1. Fuel handling system in HTR-10

2. Graphite dust test platform by lifting fuel pebble

3. Preliminary experimental data

4. Concluding remarks

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Preliminary experimental data

3 4 5 6 7 8 9 100.00

0.05

0.10

0.15

0.20

0.25

0.30

Feeding velocity (m/s)

Sta

nd

ard

dev

iati

on

(m

/s)

Standard deviations of mean lifting velocity measurement

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Preliminary experimental data

Schematic of the motion of graphite sphere during pneumatically lifting (a)ideal situation, (b)real situation in which collision occurs, (c)The axial

velocity of graphite sphere.

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3 4 5 6 7 8 9 10

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

Feeding velocity (m/s)

Abr

asio

n ra

te (

mg/

m)

The relationship between abrasion rate and average lifting velocity in airY=0.06722-0.1617x

Preliminary experimental data

mW

nS

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outline

1. Fuel handling system in HTR-10

2. Graphite dust test platform by lifting fuel pebble

3. Preliminary experimental data

4. Concluding remarks

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Concluding remarks

a test platform was designed for the study of abrasion of graphite sphere. It simulates the pebble-steel pipe abrasion of spherical fuel element during pneumatic lifting.

An automatic timing system was used to measure the mean lifting velocity, which showed a standard deviation of less than 0.2 m/s in the range of 3.5-9.5 m/s.

The relationship between abrasion rate and the mean lifting velocity was obtained on such a experimental system.

The motion mode of sphere consists of axial move along the pipe, radical oscillation, as well as rotation.

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Concluding remarks

The abrasion rate of graphite sphere increased roughly linearly with mean lifting velocity in the air, although deviation from linearity occurred at lower velocity.

The dispersion of experimental data is attributed to the large randomness of the collision between graphite sphere and the stainless steel pipe.

Further investigations can be carried out on this test platform and help the understanding of graphite dust generation in HTR-10 and HTR-PM.

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thanks